To date, shaft seal units like that depicted in FIG. 3 for example have been employed as shaft seal units used in hot water pumps or hot oil pumps such as boiler feed pumps or condensate pumps in thermal power plants.
The shaft seal unit according to the prior art depicted in FIG. 3 is furnished with a contact mechanical seal of an “inside” type (i.e., a design for sealing in a fluid that tends to leak towards the inside periphery direction from the outside periphery of the sliding faces). The shaft seal unit is designed such that a rotating-side sealing element 52 disposed next to a rotating shaft 50 that drives an internal pump impeller (not shown) and that is capable of rotating in unison with this rotating shaft 50 via a sleeve 51, and a stationary-side sealing element 55 disposed nonrotatably and capable of motive force in an axial direction on a seal cover 54 fixed to the housing 53 of the pump, are placed in intimate contact sliding at their mutually opposed end faces by a spring 56 that urges the stationary-side sealing element 55 in the axial direction. Specifically, the contact mechanical seal is intended to prevent high-temperature, high-pressure liquid inside the system (inside the pump) from leaking on the machine-exterior side from the outside periphery of the rotating shaft 50 at the mutual sliding portions S of the rotating-side sealing element 52 and the stationary-side sealing element 55.
When this type of contact mechanical seal is employed as shaft sealing means in a hot water pump or hot oil pump such as a boiler feed pump or condensate pump, the fact that the sealed fluid is at high temperature and high pressure means that the constituent components may experience deformation induced by pressure, or the high temperature sealed fluid or heat produced in the sliding portions S may give rise to thermal deformation and deterioration of materials of the constituent components, present concerns related to inconsistent sealing ability. Thus, in the prior art, some systems were configured such that a portion of the sealed fluid that flows in from inside the system and fills the space 57 to the outside periphery side of the mechanical seal is fed from a seal fluid outlet 59 of the seal cover 54 to a cooler 61 via a flushing line 60 by a partial impeller 58 formed on the sleeve 51 and rotating in unison with the rotating shaft 50, where the fluid is cooled and then recirculated into the space 57 via a flushing line 62 and a seal fluid inlet 63 of the seal cover 54, to effect cooling of the mechanical seal through this recirculation of the sealed fluid (hereinafter termed “Prior Art 1.” See Patent document 1 for example).
However, Prior Art 1 discussed above has the following problems.
(1) In order to cool the mechanical seal, it is necessary to provide flushing lines, a cooler, and a large quantity of coolant for the cooler.
(2) A pumping ring is required, and there is an associated increase in the installation space required and in power consumption. Installing a pumping ring limits the length of the inside periphery of the stuffing box and the shaft throttle portion, and therefore the cooling effect of the water cooled jacket housed within the stuffing box is insufficient.
(3) Because the mechanical seal is a single seal of an “inside balanced” type, it is necessary for the rotating-side sealing element to be installed on the outside periphery of the sleeve, increasing the size of the mechanical seal by the equivalent of the sleeve thickness. If the mechanical seal is large in size, the peripheral speed at the sliding face increases, resulting in a high load on the sliding face. Also, the sliding face, which is the heat radiating component, is located inside the stuffing box, where it cannot be afforded cooling action by the outside air.
(4) The seal end face width of the mechanical seal exceeds 2.5 mm, and due to the high heat produced by sliding, flushing is indispensible in order to eliminate the heat; this requires a large flushing flow, which therefore necessitates high flow (Q)-pump head (H) on the part of the pumping ring.
(5) In order to cool the mechanical seal, in some instances water is supplied to the sliding portions S at the outside air side thereof (in the case of FIG. 3, to the inside of the sliding portions S) to carry out quenching, but due to the high temperature, the quenching water evaporates and deposits calcium present in the water, which in some instances may lead to seizing and leaking of the mechanical seal.
Another known mechanical seal structure is an externally mounted sleeveless mechanical seal of stationary of an “outside balanced type” wherein the mechanical seal is installed on the outside of the stuffing box, having a collar directly installed on the rotating shaft and a rotating ring installed on the outside periphery of the collar, and furnished with flushing equipment (hereinafter termed “Prior Art 2”. See Patent document 2 for example).
However, Prior Art 2 also has the problem that in order to cool the mechanical seal, it is necessary to provide flushing lines, a cooler, and a large quantity of coolant for the cooler.